Systems for heating or cooling a fluid to a setpoint temperature are well known in the prior art, both in process applications and in conditioning a comfort zone. Such systems typically include a control that energizes the system if the fluid temperature deviates from the setpoint by more than a predefined first increment in one direction, and de-energizes the system if the deviation exceeds a predefined second increment in the opposite direction. The combined magnitude of these increments defines the control "deadband," i.e., a range of deviation of the fluid temperature from setpoint, usually centered around the setpoint, within which the control does not change system capacity (by turning the system on or off) in reaction to the fluid temperature error.
Larger temperature conditioning systems often have variable capacity to handle changing load conditions more efficiently. The capacity of a refrigerant compressor in such a system may be modulated using either a plurality of stages that may be selectively unloaded or de-energized, a variable speed compressor, or in the case of most systems using centrifugal compressors, by adjusting inlet guide vanes on the compressor. Control of a variable capacity system to maintain a fluid at a setpoint temperature is, of course, more complex than the simple on/off approach discussed above, however, a typical control for a variable capacity system still must include a deadband around the setpoint for control stability. The control deadband is usually selected with care. A wider deadband avoids too frequent changes in compressor capacity, while a narrower deadband provides more accurate control of the fluid temperature at the setpoint.
Selecting the width of the deadband is usually a compromise between accurate temperature control and stable operation of the system. The designer tries to avoid continually cycling a stage on and off or otherwise adjusting capacity to maintain the temperature of the fluid at the setpoint. Furthermore, he does not want the system to overshoot the setpoint as the load changes. These problems are addressed in U.S. Pat. No. 4,359,961, wherein dual deadbands are used in the control of a system having a plurality of heating and cooling stages.
The '961 patent discloses a microprocessor control in which a first narrow deadband is used to control the on/off state of whichever stage is presently cycling. Once the controlled temperature exceeds the range of a second wider deadband, the next stage (or preceding stage) is controlled to maintain the temperature within the range of the first deadband. Control of the stages within one or the other deadband is independent of whether the deviation from setpoint is increasing or decreasing, but instead depends only on the magnitude of the deviation. Thus, the second deadband essentially just defines a temperature error at which point the next stage should be energized or the preceding stage de-energized. This method does not appear to really solve the problem of minimizing setpoint overshoot while reducing cycling of the stages.
Almost every temperature conditioning system has some form of protection to prevent catastrophic failure in the event an operating parameter exceeds a safe limit, e.g., a circuit breaker to protect a compressor drive motor against damage resulting from an overload condition. On large systems, several limit switches are usually provided to shut down the system if various operating parameters exceed a safe limit. Unless there is an automatic reset, the system stays off until manually reset by an operator. Under certain load conditions, it is much more likely that an operating parameter will exceed a limit, causing the system to shut down. This can be particularly troublesome, if for example, on a very hot day when needed the most, an air conditioning system shuts down due to overload. Conversely, a problem can also occur when the system is lightly loaded and the outdoor ambient temperature is relatively cool, since under these conditions, these conditions the evaporator refrigerant temperature may drop very quickly once the system is energized, causing an evaporator refrigerant limit switch to trip to prevent the liquid being chilled in the evaporator from freezing. In both cases, the system normally shuts down and is not available for cooling until the problem is corrected and the limit switch is reset. Conventional controls modulate system capacity only to achieve and maintain the conditioned fluid at the setpoint temperature; they do not modulate capacity to avoid tripping a limit switch. On very hot days when the load exceeds the system's capacity, it is clearly more desirable to continue to operate the system to provide whatever cooling is possible than to allow an operating parameter to degrade to the point where a limit switch de-energizes the system to prevent damage.
It is therefore an object of this invention to provide a control that maintains a fluid near the setpoint temperature, yet minimizes capacity modulation.
It is a further object of this invention to provide a control that achieves the setpoint with minimal overshoot.
A still further object is to achieve the setpoint as rapidly as possible without tripping a limit switch that protects the system from catastrophic failure.
Yet a further object is to modulate system capacity to prevent an operating parameter from exceeding a safe limit, even if the setpoint temperature is not achieved by the system.
These and other objects of the invention will be apparent from the drawings and the description of the invention that follows herein below.